Water quality assessment of three tropical freshwater lakes of Kerala, SW India, with special reference to drinking water potential

Investigation on the physico-chemical parameters and bacteriological examination of surface waters of Sasthamkotta, Vellayani, and Pookot lakes of Kerala, SW India has been performed to assess the quality of lake waters for drinking purposes. For this study, a total of 60 surface water samples, 20 samples each from the three lakes were collected along S-N-W transects that cover the entire area and was performed during the year 2016–2017. The results obtained during the analyses were compared with maximum permissible limit values recommended by World Health Organization (2017) and Bureau of Indian standards (2012). The results showed that the annual concentration of a majority of the constituent parameters showed an increasing trend in their concentration.Acidification (mean pH 5.6) of Pookot lake owing to the forest canopy and spring water intrusion may deteriorate the species diversity of Pookot lake. TDS values of Sasthamkotta lake (mean TDS > 500 mg/l) and Vellayani lake (mean TDS > 500 mg/l) reflect the turbid and contamination scenario of the freshwater ecosystems. Increased concentration of primary nutrients like NO₃⁻ (mean >4 mg/l) and PO₄³⁻ (mean >3 mg/l) in Vellayani lake depict nutrient enrichment, both from anthropogenic influences and runoff from point and non-point sources in a swift pace. The observed hydro-chemical facies were NaHCO₃ for Pookot lake, NaCl with subordinates of Ca and Mg for Sasthamkotta lake and NaHCO₃ with subordinates of SO₄²⁻ for Vellayani lake, based on Piper trilinear diagrams. Water Quality Index (WQI) were calculated for evaluating influence of natural and anthropogenic activities based on several key parameters (pH, EC, TDS, HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻, Ca²⁺, Mg²⁺, Na⁺, K⁺) in surface water chemistry. As per the WQI, majority of the samples in the three lakes, based on the observed chemical constituents were falling under ‘good’ category but the real havoc was existed due to bacteriological contamination. It was observed that there was an increased count of coliforms in the lake waters of Vellayani (140 MPN/ml), Sasthamkotta (260 MPN/ml) and Pookot (90 MPN/ml), which must be zero for drinking needs, according to WHO (2017) and BIS (2012) drinking water guidelines. The increased microbial population in all the three lakes indicate the unhealthy practices in the lake catchments. Therefore, sustainable management measures should be taken to improve the water quality of the fresh water resources of Kerala.